A vestibule, foyer or entranceway with doors to the exterior on one end and doors to the interior on the other, functioning to keep indoor and outdoor air, humidity, and air temperatures separate.
A sealed, airtight chamber, such as in a manned spacecraft or submarine, used to provide access to and from the sealed area without allowing air out or water in.
A hinged or sliding door set into a floor or ceiling.
The special information that permits the inverse of a trapdoor function to be easily computed.
Such a trap set into the floor of a stage to allow fast exits and entrances.
A door in a level for regulating the ventilating current; a weather door.
A secret method of obtaining access to a program or online system; a backdoor.